Soil acidity


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This is an updated version of the presentation on soil acidity that I shared with my Soil Fertility class in fall 2010

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Soil acidity

  1. 1. Understanding Natural pH impacts optimal pH Soil Acidity In humid regions, most soils are naturally acidic but the following factors contribute to Neutral greater acidity: - parent material w/ low ANC - forest vegetation - ↑duration and intensity of chemical weathering ANC = acid neutralizing capacity Brady and Weil (2002)
  2. 2. Do plant roots really care about the H+ concentration crops Optimum pH ranges have been proposed for many in soil?
  3. 3. Collective term for the challenges faced by crops growing in acid soils The acid infertility complex
  4. 4. Nutrient availability varies with pH
  5. 5. For most soils, nutrient availability is optimized between pH 5.5 and 7.
  6. 6. Molybdenum becomes more available as pH goes up ! most ^
  7. 7. Understanding aluminum toxicity Fe and Mn toxicities also occur at lower pHs Toxic forms of Al are bioavailable at lower pHs Aluminum toxicity is minimal above a water pH of 5.5
  8. 8. Many biological processes are sensitive to aluminum toxicity
  9. 9. Crop varieties differ in their sensitivity to Al toxicity Brady and Weil, 2002
  10. 10. Multiple forms of soil acidity H+ H+ H+ H+ Soil pH is primarily a Al+3 measure of active acidity H+ H+ Active Reserve acidity acidity Sometimes called residual acidity Brady and Weil, 2002
  11. 11. Understanding pH pH = -log(H+) Brady and Weil, 2002
  12. 12. Understanding pH pH = -log(10-x) X -log(H+) neutral Brady and Weil, 2002 So what is the H+ ion concentration when the pH = 6?
  13. 13. Understanding pH pH = -log(10-x) X -log(H+) neutral Brady and Weil, 2002 So what is the pH if the H+ ion concentration is 10 x higher?
  14. 14. Understanding reserve acidity Very little lime is needed to neutralize the active acidity in soils Both soils initially have the same pH (i.e., the same amount of activity acidity) Reserve Active Reserve Active acidity acidity acidity acidity High CEC soil Low CEC soil
  15. 15. Understanding reserve acidity If only enough lime is added to deplete the active acidity, reserve acidity will quickly begin resupplying the active acidity Reserve Active Reserve Active acidity acidity acidity acidity High CEC soil Low CEC soil
  16. 16. Understanding reserve acidity More lime is needed to bring about persistent pH change in soil with more reserve acidity Effect of ΔpH adding the same rate ΔpH of lime to soils with different amounts of reserve acidity Reserve Active Reserve Active acidity acidity acidity acidity High CEC soil Low CEC soil
  17. 17. Each charge depicted on this diagram represents 1 centimol of charge per kg of soil K+ Sum of non-acid cations -- _____________________ -- %BS = Ca+2 * 100 Sum of all cations - Mg+2 Humus - H+ H20 H20 Exchangeable exchangeable soil H20 acidity cations solution H20 H20 - H20 - Al Clay -- K +3 + H2O ↔ Al(OH)3 + 3H+ + SO4 -2 + -- + What is the “base” Ca+2 saturation of this soil ?
  18. 18. Is pH related to base saturation ? 100 80 60 40 20 0 Acid Saturation, %
  19. 19. Isis probably more accuratesaturation ? It pH related to base to say that active acidity is related to acid saturation 100 80 60 40 20 0 Acid Saturation, %
  20. 20. pH dependent charge In contrast, the charge on humus is higher at higher pHs The dominant clay minerals in IL have mostly permanent charge created by isomorphic substitution
  21. 21. The charge on humic substances (and low activity clays) is very pH dependent H+ H+ H+ H+ H+ ions dissociate when the soil pH increases and reassociate when the pH drops. Brady and Weil (2002)
  22. 22. Soil acidity increases when H+ producing processes exceed H+ consuming processes.
  23. 23. Many processes add H+ ions to soils 1) Carbonic acid forms when carbon dioxide dissolves in water. H+ ions are released when carbonic acid dissociates: H2CO3 → HCO3- + H+ VERY IMPORTANT PART OF SOIL FORMATION 2) Organic acids form during the decomposition of organic matter. H+ ions are released when these organic acids dissociate. 3) Sulfuric and nitric acids form during the oxidation of reduced forms of N and S (e.g., NH4+ from fertilizer). Nitrification NH4+ + 2O2 → NO3- + 2H+ + H2O 4) Sulfuric and nitric acids form when sulfur oxides and nitric oxides (released into the atmosphere by automobile emissions, industry smoke stacks, volcanoes, forest fires) dissolve in precipitation. H2SO4 and HNO3 are strong acids and fully dissociate in water. 5) Roots release H+ to balance internal charge when cation uptake exceeds anion uptake.
  24. 24. K+ H+ - The pH of a plant’s NO3 rhizosphere changes as the plant regulates OH- its internal charge balance.
  25. 25. Which plant received nitrate (NO3-)? Which plant received ammonium (NH4+)?
  26. 26. Many processes consume H+ ions in soils 1) Weathering of most minerals (e.g., silicates, carbonates…) 2) Decomposition of organic anions 3) Reduction of oxidized forms of N, S and Fe. 4) Roots release OH- or HCO3- to balance internal charge when anion uptake exceeds cation uptake 5) Inner sphere adsorption of anions (especially sulfate) which displaces hydroxyl (OH-) groups
  27. 27. Acidity What is liberated and what is left behind when plant biomass is burned ? Oxides of C, N and S Elements that Oxides of have traditionally Alkalinity Ca, Mg and K been called “bases”
  28. 28. C, N and S oxides cause acid precipitation Brady and Weil, 2002
  29. 29. Forest damaged by acid rain
  30. 30. Forest damaged by acid rain Looks great but may be devoid of life if acid rain has created Al toxicity
  31. 31. Forest damaged by acid rain Looks great but may be devoid of life Monument getting if acid rain has created Al by acid rain dissolved toxicity
  32. 32. Sliding down the acidity slope The effect of added acidity on Carbonates soil pH depends on the soil’s buffer capacity Chadwick and Chorover ( 2001)
  33. 33. Acid inputs promote leaching of non-acid cations Nitric acid = HNO3 → NO3- + H+ Why does leaching of these anions cause soil acidification ? Brady and Weil, 2002
  34. 34. released into the soil 1H+ consumed Nitrification is an acidifying process, right?? 1H+ NH3 consumed
  35. 35. Complete N cycle (no net acidification) released into the soil 1H+ consumed 1H+ NH3 consumed The 2 H+ produced during nitrification are balanced by 2 H+ consumed during the formation of NH4+ and the uptake of NO3- by plants
  36. 36. Very important in places where lime is expensive!
  37. 37. Standard values for the quantity of lime needed to neutralize the acidity generated by specific N fertilizers Assumes: 1) all ammonium-N is converted to nitrate-N and 2) half of the nitrate is leached. Lime required Nitrogen source Composition (lb CaCO3 / lb N) Anhydrous ammonia 82-0-0 1.8 Urea 46-0-0 1.8 Ammonium nitrate 34-0-0 1.8 Ammonium sulfate 21-0-0-24 5.4 Monoammonium 10-52-0 5.4 phosphate Diammonium 18-46-0 3.6 phosphate
  38. 38. Harvest of crop biomass removes alkalinity from agricultural fields Lime required to replace alkalinity Cation : N ratio Crop removed in harvest in plant biomass (lb CaCO3 /100 lb of N harvested) Corn grain 0.14 25 Corn stover 0.73 131 Soybean 0.14 25 Oats grain 0.14 25 Oats straw 0.94 169 Alfalfa 1.41 254
  39. 39. Scenario Corn/soybean rotation 200 bu corn, 50 bu soybeans All P supplied as DAP N applied as DAP and AA Acidity from N fertilizer 3.6 x 52 lbs of N in DAP required to ~ 190 lbs of lime supply P removed in harvest 1.8 x 150 lbs of N in AA ~ 270 lbs of lime Acidity from grain harvest 25 x 180 lbs of N harvested/100 ~ 45 lbs of lime 25 x 200 lbs of N harvested/100 ~ 50 lbs of lime Projected lime requirement ~ 0.3 tons/rotation
  40. 40. In many parts of the world, notably the US Midwest and Europe, soils are often limed to a near neutral pH 6.5–7.0. Because plants do not directly respond to H+ concentration, it is pertinent to inquire why this approach to liming has enjoyed such widespread popularity. The original near-neutral pH of many of the soils was no doubt a consideration as was the use of acid-sensitive forage legumes to supply N in rotations during the era when the original lime experiments were conducted. The introduction of the pH meter at about the same time as N fertilizers found widespread popularity (replacing forage legumes in rotations) facilitated measurement of soil acidity and removed the focus from the real problems of soil acidity, namely, toxic levels of Al and Mn and deficiencies of nutrients such as Ca, Mg, N, S, P and Mo. Even after forage legumes disappeared from most rotations, high target pH values were retained.
  41. 41. Liming experiments throughout the world reveal that, with very few exceptions, all grain crops including legumes cease to respond to lime above pH 5:5–5:8; provided that the nutrients (Ca, Mg, Mo, B, P, etc) negatively impacted by soil acidity are optimized. On highly weathered soils (e.g., NC and Brazil), liming to near neutrality can have disastrous effects on yields of many crops. Many examples are presented in the article of the few benefits of liming to neutrality and the many benefits of farming with levels of acidity somewhat more intense than has normally been the case. Among the latter benefits are increased profitability from higher nutrient efficiencies, reduced diseases and pests, slower nitrification with less water pollution, improved soil tilth, improved availability of many metals and P.
  42. 42. Do you remember this graph? Impact of pH and an inhibitor on % nitrification % Nitrification w/ N serve Soil pH
  43. 43. According to Sumner: Ever since pH became an easily measured soil parameter (invention of the pH meter), we have focused on an indicator of soil acidity (pH) rather than on the actual plant limiting factors associated with acidity (toxicities, deficiencies and imbalances).
  44. 44. Alfalfa field with dead strip where lime was not applied How should lime rates be determined?
  45. 45. Lime rates should be guided by soil testing
  46. 46. Pocket pH meters can be very useful but require regular calibration !!!
  47. 47. Sources of variation in soil pH measurements 1. The soil to solution ratio used when measuring pH. 2. The salt content of the diluting solution used to achieve the desired soil to solution ratio. 3. The carbon dioxide content of the soil and solution. 4. Errors associated with standardization of the instrument used to measure pH.
  48. 48. Water pH > Salt pH Salt solutions are normally used when measuring the pH of soils in arid regions (i.e. places where salinity is often an issue) Brady and Weil, 2002
  49. 49. Soil pH depends on the method used to measure it !! As a result, the method of measurement should be reported whenever soil pH data is discussed.
  50. 50. The amount of lime needed to bring about a 1 unit change in pH varies widely between soils
  51. 51. When a soil is limed, Ca+2 from the lime displaces exchangeable acidity from the soil colloids. The active acidity that is generated reacts with the carbonate ions from the lime, producing water and carbon dioxide. H+ Ca+2 soil colloid + CaCO3 soil colloid + H2O + CO2 H+
  52. 52. “Illinois method” of determining lime requirement How do you know which line to use ? The lines represent different levels of reserve acidity Steeper line = more reserve acidity
  53. 53. Choosing the right line Line A: Dark colored silty clays and silty clay loams (CEC > 24) Line B: Light and medium colored silty clays and silty clay loams, dark colored silts and clay loams (CEC 15-24) Line C: Light and medium colored silt and clay loams, dark and medium colored loams, dark colored sandy loams (CEC 8-15) Line D: Light colored loams, light and medium colored sandy loams and all sands (CEC < 8) Line E: Mucks and peat (organic soils). Light colored (< 2.5% OM) Medium colored (2.5-4.5% OM) Dark colored (4.5% OM)
  54. 54. “Buffer pH” is a measure of reserve acidity
  55. 55. Not all limestone is the same ! Pure calcium carbonate has a calcium carbonate equivalency (CCE) of 100 and is the standard against which all liming materials are compared. A ton of material with a CCE of 90 % can neutralize 10% less acid than a ton of pure calcium carbonate. Liming materials that are finely ground, have more surface area in contact with the soil solution than coarser ground materials and thus will neutralize soil acidity more rapidly. Fineness of grind is rated according to the percentage of material that will pass through 8-, 30-, and 60-mesh screens.
  56. 56.
  57. 57. Page from the 2008 IL Lime book Multiply by these factors
  58. 58. Adjusting for differences in lime particle size distribution
  59. 59. Lime requirements determined using the “Illinois method” assume the following: A. A 9-inch tillage depth. If tillage is less than 9 inches, reduce the amount of limestone; if more than 9 inches, increase the lime rate proportionately. In no-till systems, use a 3-inch depth for calculations (one-third the amount suggested for soil moldboard-plowed 9 inches deep). B. Typical fineness of limestone. Ten percent of the particles are greater than 8-mesh; 30 percent pass an 8-mesh and are held on 30- mesh; 30 percent pass a 30-mesh and are held on 60-mesh; and 30 percent pass a 60-mesh. C. A calcium carbonate equivalent (total neutralizing power) of 90 percent. The rate of application may be adjusted according to the deviation from 90. Lime rates should be adjusted if any of these assumptions are not accurate
  60. 60. It takes time for lime to react in soil
  61. 61. pH measurements on the fly Soil pH often varies widely within fields Don’t forget that some measure of OM, CEC or clay content is also needed to make a variable rate lime map.
  62. 62. Both past management and inherent soil properties affect soil pH and lime requirement Why is variable rate lime more likely to pay than variable rate N, P or K?
  63. 63. Both past management and inherent soil properties affect soil pH and lime requirement Over-liming and under- Why is variable rate lime liming likely tonegative more have pay than variable rate N, P or K? effects on yield
  64. 64. Insufficient lime is applied to neutralize total acid inputs to IL soils South eastern IL has fewer quarries and the greatest lime deficit
  65. 65. Barak P, Jobe BO, Krueger AR, Peterson LA, Laird DA 1997. Effects of long- term soil acidification due to nitrogen fertilizer inputs in Wisconsin. PLANT AND SOIL. 197(1): 61-69 Abstract: Agroecosystems are domesticated ecosystems intermediate between natural ecosystems and fabricated ecosystems, and occupy nearly one-third of the land areas of the earth. Chemical perturbations as a result of human activity are particularly likely in agroecosystems because of the intensity of that activity, which include nutrient inputs intended to supplement native nutrient pools and to support greater biomass production and removal. At a long-term fertility trial in South-Central Wisconsin, USA, application of ammoniacal N fertilizer resulted in significant increases in exchangeable acidity accompanied by decreases in cation exchange capacity (CEC), base saturation, and exchangeable Ca2+ and Mg2+ . Plant analysis shows that a considerable portion of the alkalinity generated by assimilation of N (and to a lesser extent by S) is sequestered in the above-ground plant parts as organic anions and is not returned to the soil if harvested. Elemental analysis of soil clays indicates a loss of 16% of the CEC. The reversibility of this change is doubtful if the changes are due to weathering of soil minerals.
  66. 66. Summary of common soil fertility problems that rarely occur in soils with pHs between 5.5 and 7 pH < 5.5 pH > 7.0 Al toxicity to plant roots Fe deficiency Mn toxicity to plant roots Mn deficiency Ca and Mg deficiency Zn deficiency Mo deficiency in legumes *Osmotic stress from salts P tied up by Fe and Al P tied up by Ca and Mg Slow N transformations Potato scab
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